1. Field of the Invention
The present invention relates to a photomask used in photolithography which is one of the processes of manufacturing a semiconductor integrated circuit, and more particularly, to a photomask which can contribute to the improvement of process latitude in photolithography.
2. Description of the Related Art
FIG. 7 schematically illustrates an optical system of an optical stepper used in photolithography which is one of the manufacturing processes of a semiconductor device. The optical stepper is an apparatus for reduction-projecting a pattern of a photomask onto a semiconductor substrate. Such an optical stepper includes an illumination optical system from the pattern of the photomask to a light source, and a projection optical system from the pattern on the photomask to the semiconductor substrate. The pattern of the photomask is transferred to a whole semiconductor wafer in a step-and-repeat manner.
The stepper of FIG. 7 is provided with a mercury lamp 10 as a light source. KrF excimer laser can be used instead of the mercury lamp, if desired. A fly-eye lens 11 acts to approximate light radiated from a point light source 10 to light radiated from a planar light source. A stop is placed in a pupil (Fourier plane) 12 of the illumination optical system. A pattern of a photomask 1 is illuminated with light passing through a condenser lens 13. The light passing through the pattern of photomask 1 is focused on a semiconductor wafer 16 by a reduction-projection lens 14. A stop is also placed in a pupil 15 of the projection optical system.
FIG. 8 is a schematic cross section of a conventional photomask used in the optical stepper shown in FIG. 7. Photomask 1 includes a transparent substrate 2. Transparent substrate 2 has two main surfaces 2a and 2b smoothly polished and opposite to each other. A light-shielding pattern 3 is formed on one main surface 2a. Light-shielding pattern 3 can be formed, for example, of a Cr film or a complex film of a Cr film and a CrOx film.
When degree of integration of a semiconductor integrated circuit is enhanced, the improvement of resolution and of the depth of focus in photolithography is desired. If a wavelength of light used in photolithography is made shorter, resolution and the depth of focus may both be improved. In addition, resolution tends to be improved in proportion to an increase in a numerical aperture NA of the optical stepper. However, the depth of focus tends to be reduced in inverse proportion to the square of the numerical aperture NA. Accordingly, when a pattern is transferred with reduced depth of focus to a wafer having a difference in level, which is under the manufacturing process of the semiconductor integrated circuit, sufficient contrast can be obtained only at an upper level or a lower level of the wafer, resulting in low process latitude. Therefore, it is important to prevent reduction in the depth of focus, when the process latitude of the optical stepper is to be enhanced.
In order to achieve greater depth of focus without reducing resolution in the optical stepper, it is desired to make a coherence value .sigma. larger. The relationship between the coherence value .sigma. and the depth of focus is described, for example, in SPIE, Vol. 1927, pp. 310-319 (1993) by Yamanaka et al. The coherence value .sigma. is defined by the following equation (1) EQU .sigma.=NA.sub.1 /NA.sub.2 ( 1)
where NA.sub.1 is the numerical aperture of the illumination optical system and NA.sub.2 is the numerical aperture of the projection optical system.
The numerical aperture NA.sub.2 of the projection optical system is desired to be as large as possible in order to achieve improved resolution. The maximum numerical aperture NA.sub.2 of the projection optical system in the optical stepper is, however, about 0.6, due to the limitation of diameter and efficiency of a lens used. In the optical stepper, the numerical aperture NA.sub.2 of the projection optical system is maintained to a desired constant value. Accordingly, in order to achieve larger coherence value .sigma., the numerical aperture NA.sub.1 of the illumination optical system is required to be made larger. The numerical aperture NA.sub.1 of the illumination optical system is determined mainly by the size of the aperture in the stop placed in pupil 12.
In FIG. 9, an example of the stop placed in pupil 12 of the illumination optical system is schematically illustrated. The stop of FIG. 9 is used in the normal illumination, and includes a light-shielding circular plate 12A and a circular aperture 12a provided in the center thereof. Therefore, illumination light passes through aperture 12a in a stop 12A, and the value of the numerical aperture NA.sub.1 of the illumination system is determined mainly by the size of diameter of aperture 12a. In other words, the coherence value .sigma. can be increased by making aperture 12a in stop 12A larger. Thus, the depth of focus is improved by the increased value of .sigma., resulting in the improvement of process latitude upon projecting light on the semiconductor substrate. However, the increase in the diameter of aperture 12a in stop 12A is limited by diameter and efficiency of the lens used in the illumination optical system.
In recent years, off-axis illumination has been studied in order to achieve greater depth of focus in the optical stepper. The principle of the off-axis illumination is described, for example, in SPIE, Vol. 1927, pp. 103-123 (1993) by Luehrmann et al.
FIGS. 10 and 11 show examples of a stop used in pupil 12 of the illumination optical system in the off-axis illumination. The stop of FIG. 10 is used when a mask pattern including a plurality of equally spaced parallel lines are transferred to the semiconductor substrate. Then, the stop is placed such that a line linking the centers of two apertures 12b in stop 12B is perpendicular to the lines of the mask pattern. Four apertures 12c in point symmetry with respect to the center of light-shielding circular plate 12C are located in the stop of FIG. 11. This stop is used in the off-axis illumination for transferring a two-dimensional fine pattern.
As described above, since coherency of conventional illumination light is controlled by regular spacial positioning of the apertures in the stop, the effect of improving contrast of projected image is obtained for a specific pattern suitable for the regular spacial positioning of the apertures. However, it has adverse effects such as distortion of image and reduction of contrast on a pattern not suitable for the regular spacial positioning of the apertures.
FIG. 12 is a graph showing the relation between a line width transferred and the depth of focus in the off-axis illumination. In this graph, the abscissa indicates the line width and the ordinate indicates the depth of focus. Curve A indicates the depth of focus in the off-axis illumination using the stop shown in FIG. 10, while dashed curve P indicates the depth of focus in the normal illumination using the stop shown in FIG. 9. As understood from FIG. 12, in the off-axis illumination, greater depth of focus is obtained for a pattern having a specific line width. For a pattern having a different line width from the specific one, however, the depth of focus in the off-axis illumination is smaller than that in the normal illumination. The effect of the relation between positioning of the apertures in the stop used in this off-axis illumination and a mask pattern on the depth of focus is described, for example, in SPIE, Vol. 1927, pp. 190-213. That is, in the off-axis illumination, there is a problem that the depth of focus can be improved only for a specific pattern and not for all the patterns.